1. Field of the Invention
The present invention relates to an input to output isolated DC (direct current) to DC converter.
2. Description of the Related Art
Input/output isolated DC to DC converters each with a transformer are capable of isolating between input and output thereof, and they have been widely used. Some DC to DC converters each with a transformer whose secondary coil consists of two secondary windings are disclosed in the following publications:
(1) U.S. Pat. No. 5,291,382 corresponding to JP Unexamined Patent Publication No. H5-276751;
(2) U.S. Pat. No. 3,175,388 corresponding to JP Patent Publication No. 3175388;
(3) JP Unexamined Patent Publication No. 2003-79142;
(4) U.S. Pat. No. 6,400,579 corresponding to National Publication of Translated Vernon No. 2003-529311;
(5) U.S. Pat. No. 6,304,460 corresponding to National Publication of Translated Version No. 2003-533163;
(6) U.S. Pat. No. 6,462,962 corresponding to National Publication of Translated Version No. 2004-508799;
(7) JP Unexamined Patent Publication No. 2003-102175; and
(8) U.S. Pat. No. 5,291,382
In these publications, note that a type of secondary coils consisting of one secondary winding with several tappings is included in the type of secondary coils each with two secondary windings.
A full-wave rectifier with two rectifying elements is preferably used as a secondary rectifier in such types of DC to DC converters. The full-wave rectifier converts an AC (Alternating Current) voltage induced across the secondary coil into a DC voltage using both half cycles of the AC voltage, thereby delivering it as a secondary current toward an electrical load.
Specifically, the full-wave rectifier uses two rectifying elements of which one conducts during one half cycle while the other conducts during the other half cycle of the induced AC voltage.
The DC to DC converters, the transformer of each of which is provided with two secondary windings, have an advantage that allows, when one of the different rectifying elements converts the AC voltage induced across one of the two secondary windings into the DC voltage during one half cycle, the other of the different rectifying elements to serve as a choke coil. This permits current ripples and losses caused thereby to decrease.
In addition, there is disclosed a DC to DC converter with one transformer and a full-wave rectifier with different rectifying elements; a single core of this transformer is wound with two secondary windings in the same direction. One of the rectifying elements converts an AC voltage induced across one of the secondary windings into a DC voltage during one half cycle of the AC voltage; the other of the rectifying elements converts the AC voltage induced across the other of the two secondary windings into a DC voltage during the other half cycle thereof.
The secondary coil and the readying elements of a transformer of such a DC to DC converter set forth above are configured to alternately generate and output a secondary current toward an electrical load during one half cycle of an input AC voltage, and a secondary current to the electrical load during the other half cycle of the input AC voltage.
This structure of the DC to DC converter may cause significant change in curt at each of the secondary windings and wires leading from the output terminal thereof to the electrical load during a period around which one and the other half cycles are switched; this period is referred to as a switching transient period.
This means that high frequency current components may flow through and out of the wires during the switching transient period, so that significant electromagnetic waves may be generated from the wires. The significant electromagnetic wave generation may become a major issue for step-down converters.
The high frequency current components flowing through the wires during the switching transient period may generate wasted capacitive leak currents and/or inductive surge voltages through AC impedance components in the secondary circuit portion of the transformer.